1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
|
Transport Layer Security (TLS)
========================================
.. versionadded:: 1.11.0
Botan supports both client and server implementations of the SSL/TLS
protocols, including SSL v3, TLS v1.0, TLS v1.1, and TLS v1.2 (the
insecure and obsolete SSL v2 protocol is not supported, beyond
processing SSL v2 client hellos which some clients still send for
backwards compatability with ancient servers).
The TLS implementation does not know anything about sockets or
networks. Instead, it calls a user provided callback (hereafter
``output_fn``) whenever it has data that it would want to send to the
other party, and whenever the application receives some data from the
counterparty it passes that information to TLS using
:cpp:func:`TLS::Channel::received_data`. If the data passed in results
in a handshake completing, then the user provided
``handshake_complete`` is called, and if some application data being
received, or a TLS :ref:`alert <tls_alerts>` is received, the another
user provided callback, hereafter ``proc_fn``, is called.
The callbacks that TLS calls have the signatures
.. cpp:function:: void output_fn(const byte data[], size_t data_len)
TLS requests that all bytes of *data* be queued up to send. After
this function returns, *data* will be overwritten, so a copy
should be made if the callback cannot send the immediately.
.. cpp:function:: void proc_fn(const byte data[], size_t data_len, \
const TLS::Alert& alert)
Called whenever application data or an alert is received from the
other side of the connection, in which case *data* and *data_len*
specify the data received. This array will be overwritten
sometime after the callback returns, so again a copy should be
made if need be.
If :cpp:func:`TLS::Alert::is_valid` is true then *alert*
specifies a valid alert that was received. Note that if received
before the handshake has completed, an attacker can easily insert
false alert values.
.. note::
Currently, if an alert was passed in then no other data was
provided. This allows an appliction to easily distinguish
between data sent before and after the alert was received.
.. cpp:function:: bool handshake_complete(const TLS::Session& session)
Called whenever a negotiation completes. This can happen more
than once on any connection. The *session* parameter provides
information about the session which was established.
If this function returns false, the session will not be cached
for later resumption.
If this function wishes to cancel the handshake, it can throw an
exception which will send a close message to the counterparty and
reset the connection state.
You can of course use tools like ``std::bind`` to bind additional
parameters to your callback functions.
TLS Channels
----------------------------------------
TLS servers and clients share an interface called `TLS::Channel`. A
TLS channel (either client or server object) has these methods
available:
.. cpp:class:: TLS::Channel
.. cpp:function:: size_t received_data(const byte buf[], size_t buf_size)
This function is used to provide data sent by the counterparty
(eg data that you read off the socket layer). Depending on the
current protocol state and the amount of data provided this may
result in one or more callback functions that were provided to
the constructor being called.
The return value of ``received_data`` specifies how many more
bytes of input are needed to make any progress, unless the end of
the data fell exactly on a message boundary, in which case it
will return 0 instead.
.. cpp:function:: void send(const byte buf[], size_t buf_size)
If the connection has completed the initial handshake process,
the data provided is sent to the counterparty as TLS
traffic. Otherwise, an exception is thrown.
.. cpp:function:: void close()
A close notification is sent to the counterparty, and the
internal state is cleared.
.. cpp:function:: bool is_active()
Returns true if and only if a handshake has been completed on
this connection and the connection has not been subsequently
closed.
.. cpp:function:: bool is_closed()
Returns true if and only if either a close notification or a
fatal alert message have been either sent or received.
.. cpp:function:: void renegotiate(bool force_full_renegotiation = false)
Initiates a renegotiation. The counterparty is allowed by the
protocol to ignore this request. If a successful renegotiation
occurs, the *handshake_complete* callback will be called again.
If *force_full_renegotiation* is false, then the client will
attempt to simply renew the current session - this will refresh
the symmetric keys but will not change the session master
secret. Otherwise it will initiate a completely new session.
For a server, if *force_full_renegotiation* is false, then a
session resumption will be allowed if the client attempts
it. Otherwise the server will prevent resumption and force the
creation of a new session.
.. cpp:function:: std::vector<X509_Certificate> peer_cert_chain()
Returns the certificate chain of the counterparty. When acting
as a client, this value will be non-empty unless the client's
policy allowed anonymous connections and the server then chose
an anonymous ciphersuite. Acting as a server, this value will
ordinarily be empty, unless the server requested a certificate
and the client responded with one.
.. _tls_client:
TLS Clients
----------------------------------------
.. cpp:class:: TLS::Client
.. cpp:function:: TLS::Client( \
std::function<void, const byte*, size_t> output_fn, \
std::function<void, const byte*, size_t, TLS::Alert> proc_fn, \
std::function<bool, const TLS::Session&> handshake_complete, \
TLS::Session_Manager& session_manager, \
Credentials_Manager& credendials_manager, \
const TLS::Policy& policy, \
RandomNumberGenerator& rng, \
const std::string& servername = "", \
std::function<std::string, std::vector<std::string> > next_protocol)
Initialize a new TLS client. The constructor will immediately
initiate a new session.
The *output_fn* callback will be called with output that
should be sent to the counterparty. For instance this will be
called immediately from the constructor after the client hello
message is constructed. An implementation of *output_fn* is
allowed to defer the write (for instance if writing when the
callback occurs would block), but should eventually write the data
to the counterparty *in order*.
The *proc_fn* will be called with data sent by the counterparty
after it has been processed. The byte array and size_t represent
the plaintext; the :cpp:class:`TLS::Alert` value provides
notification if the counterparty sent an alert via the TLS alert
system. Possible values of alert data are included in the
Alert_Type enum. Particularly relevant is the CLOSE_NOTIFY value.
The *handshake_complete* function is called when a handshake
(either initial or renegotiation) is completed. The return value of
the callback specifies if the session should be cached for later
resumption. If the function for some reason desires to prevent the
connection from completing, it should throw an exception
(preferably a TLS::Exception, which can provide more specific alert
information to the counterparty). The :cpp:class:`TLS::Session`
provides information about the session that was just established.
The *session_manager* is an interface for storing TLS sessions,
which allows for session resumption upon reconnecting to a server.
In the absence of a need for persistent sessions, use
:cpp:class:`TLS::Session_Manager_In_Memory` which caches
connections for the lifetime of a single process. See
:ref:`tls_session_managers` for more about session managers.
The *credentials_manager* is an interface that will be called to
retrieve any certificates, secret keys, pre-shared keys, or SRP
intformation; see :doc:`credentials_manager` for more information.
Use *servername* to specify the DNS name of the server you are
attempting to connect to, if you know it. This helps the server
select what certificate to use and helps the client validate the
connection.
The optional *next_protocol* callback is called if the server
indicates it supports the next protocol notification extension.
The callback wlil be called with a list of protocol names that the
server advertises, and the client can select from them or return an
unadvertised protocol.
A simple TLS client example:
.. literalinclude:: examples/tls_client.cpp
TLS Servers
----------------------------------------
.. cpp:class:: TLS::Server
.. cpp:function:: TLS::Server( \
std::function<void, const byte*, size_t> output_fn, \
std::function<void, const byte*, size_t, TLS::Alert> proc_fn, \
std::function<bool, const TLS::Session&> handshake_complete, \
TLS::Session_Manager& session_manager, \
Credentials_Manager& creds, \
const TLS::Policy& policy, \
RandomNumberGenerator& rng, \
const std::vector<std::string>& protocols)
The first 7 arguments are treated similiarly to the :ref:`client
<tls_client>`. The final (optional) argument, protocols, specifies
the protocols the server is willing to advertise it supports.
A TLS server that can handle concurrent connections using asio:
.. literalinclude:: examples/asio_tls_server.cpp
.. _tls_sessions:
TLS Sessions
----------------------------------------
TLS allows clients and servers to support *session resumption*, where
the end point retains some information about an established session
and then reuse that information to bootstrap a new session in way that
is much cheaper computationally than a full handshake.
Every time your handshake callback is called, a new session has been
established, and a ``TLS::Session`` is included that provides
information about that session:
.. cpp:class:: TLS::Session
.. cpp:function:: Protocol_Version version() const
Returns the protocol version that was negotiated
.. cpp:function:: Ciphersuite ciphersite() const
Returns the :ref:`ciphersuite <tls_ciphersuite>` that was negotiated.
.. cpp:function:: std::string sni_hostname() const
Returns the hostname the client indicated in the hello message.
.. cpp:function:: std::vector<X509_Certificate> peer_certs() const
Returns the certificate chain of the peer
.. cpp:function:: std::string srp_identifier() const
If an SRP ciphersuite was used, then this is the identifier
that was used for authentication.
.. cpp:function:: bool secure_renegotiation() const
Returns ``true`` if the connection was negotiated with the
correct extensions to prevent the renegotiation attack.
There are also functions for serialization and deserializing sessions:
.. cpp:class:: TLS::Session
.. cpp:function:: std::vector<byte> encrypt(const SymmetricKey& key, \
RandomNumberGenerator& rng)
Encrypts a session using a symmetric key *key* and returns a raw
binary value that can later be passed to ``decrypt``.
.. cpp:function:: static Session decrypt(const byte ciphertext[], \
size_t length, \
const SymmetricKey& key)
Decrypts a session that was encrypted previously with
``encrypt`` and *key*, or throws an exception if decryption
fails.
.. cpp:function:: secure_vector<byte> DER_encode() const
Returns a serialized version of the session.
.. warning:: The return value contains the master secret for
the session, and an attacker who recovers it could
recover plaintext of previous sessions or
impersonate one side to the other.
.. _tls_session_managers:
TLS Session Managers
----------------------------------------
You may want sessions stored in a specific format or storage type. To
do so, implement the ``TLS::Session_Manager`` interface and pass your
implementation to the ``TLS::Client`` or ``TLS::Server`` constructor.
.. cpp:class:: TLS::Session_Mananger
.. cpp:function:: void save(const Session& session)
Save a new *session*. It is possible that this sessions session
ID will replicate a session ID already stored, in which case the
new session information should overwrite the previous information.
.. cpp:function:: void remove_entry(const std::vector<byte>& session_id)
Remove the session identified by *session_id*. Future attempts
at resumption should fail for this session.
.. cpp:function:: bool load_from_session_id(const std::vector<byte>& session_id, \
Session& session)
Attempt to resume a session identified by *session_id*. If
located, *session* is set to the session data previously passed
to *save*, and ``true`` is returned. Otherwise *session* is not
modified and ``false`` is returned.
.. cpp:function:: bool load_from_host_info(const std::string& hostname, u16bit port, \
Session& session)
Attempt to resume a session for *hostname* / *port*. If *port*
is zero, try to find a session for *hostname* on any port. With
the current client implementation, *port* is always zero.
.. cpp:function:: std::chrono::seconds session_lifetime() const
Returns the expected maximum lifetime of a session when using
this session manager. Will return 0 if the lifetime is unknown
or has no explicit expiration policy.
.. _tls_session_manager_inmem:
In Memory Session Manager
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The ``TLS::Session_Manager_In_Memory`` implementation saves sessions
in memory, with an upper bound on the maximum number of sessions and
the lifetime of a session.
.. cpp:class:: TLS::Session_Manager_In_Memory
.. cpp:function:: Session_Manager_In_Memory(size_t max_sessions = 1000, \
std::chrono::seconds session_lifetime = 7200)
Limits the maximum number of saved sessions to *max_sessions*, and
expires all sessions older than *session_lifetime*.
Noop Session Mananger
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The ``TLS::Session_Manager_Noop`` implementation does not save
sessions at all, and thus session resumption always fails. Its
constructor has no arguments.
SQLite3 Session Manager
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This session manager is only available if support for SQLite3 was
enabled at build time. If the macro
``BOTAN_HAS_TLS_SQLITE3_SESSION_MANAGER`` is defined, then
``botan/tls_session_manager_sqlite.h`` contains
``TLS::Session_Manager_SQLite`` which stores sessions persistently to
a sqlite3 database. The session data is encrypted using a passphrase,
and stored in two tables, named ``tls_sessions`` (which holds the
actual session information) and ``tls_sessions_metadata`` (which holds
the PBKDF information).
.. warning:: The hostnames associated with the saved sessions are
stored in the database in plaintext. This may be a
serious privacy risk in some applications.
.. cpp:class:: TLS::Session_Manager_SQLite
.. cpp:function:: Session_Manager_SQLite( \
const std::string& passphrase, \
RandomNumberGenerator& rng, \
const std::string& db_filename, \
size_t max_sessions = 1000, \
std::chrono::seconds session_lifetime = 7200)
Uses the sqlite3 database named by *db_filename*.
TLS Policies
----------------------------------------
``TLS::Policy`` is how an application can control details of what will
be negotiated during a handshake.
.. cpp:class:: TLS::Policy
.. cpp:function:: Protocol_Version min_version() const
Returns the minimum protocol version we are willing to negotiate.
Default: SSL v3
.. cpp:function:: Protocol_Version pref_version() const
Return the protocol version we would prefer to negotiate. This is
the version that clients will offer to servers.
Default: TLS v1.2
.. cpp:function:: std::vector<std::string> allowed_ciphers() const
Returns the list of ciphers we are willing to negotiate, in order
of preference.
Default: "AES-256", "AES-128", "ARC4", "3DES"
Also allowed: "Camellia-256", "Camellia-128", "SEED"
.. cpp:function:: std::vector<std::string> allowed_macs() const
Returns the list of algorithms we are willing to use for
message authentication, in order of preference.
Default: "SHA-384", "SHA-256", "SHA-1"
Also allowed: "MD5"
.. cpp:function:: std::vector<std::string> allowed_key_exchange_methods() const
Returns the list of key exchange methods we are willing to use,
in order of preference.
Default: "ECDH", "DH", "RSA"
Also allowed: "SRP_SHA", "ECDHE_PSK", "DHE_PSK", "PSK"
.. cpp:function:: std::vector<std::string> allowed_signature_hashes() const
Returns the list of algorithms we are willing to use for
public key signatures, in order of preference.
Default: "SHA-512", "SHA-384", "SHA-256", "SHA-224", "SHA-1"
Also allowed (although **not recommended**): "MD5"
.. note::
This is only used with TLS v1.2. In earlier versions of the
protocol, signatures are fixed to using only SHA-1 (for
DSA/ECDSA) or a MD5/SHA-1 pair (for RSA).
.. cpp:function:: std::vector<std::string> allowed_signature_methods() const
Default: "ECDSA", "RSA", "DSA"
Also allowed: "" (meaning anonymous)
.. cpp:function:: std::vector<std::string> allowed_ecc_curves() const
Return a list of ECC curves we are willing to use, in order of preference.
Default: "secp521r1", "secp384r1", "secp256r1",
"secp256k1", "secp224r1", "secp224k1"
Also allowed: "secp192r1", "secp192k1", "secp160r2", "secp160r1", "secp160k1"
.. cpp:function:: std::vector<byte> compression() const
Return the list of compression methods we are willing to use, in order of
preference.
.. note::
TLS compression is not currently supported.
.. cpp:function:: bool allow_insecure_renegotiation() const
If this function returns true, we will allow renegotiation attempts
even if the counterparty does not support the RFC 5746 extensions.
.. warning:: Returning true here could expose you to attacks
Default: false
.. cpp:function:: DL_Group dh_group() const
For ephemeral Diffie-Hellman key exchange, the server sends a
group parameter. Return the group parameter a server should
use.
Default: 2048 bit IETF IPsec group ("modp/ietf/2048")
.. cpp:function:: size_t minimum_dh_group_size() const
Return the minimum size in bits for a Diffie-Hellman group that a
client will accept. Due to the design of the protocol the client
has only two options - accept the group, or reject it with a
fatal alert then attempt to reconnect after disabling ephemeral
Diffie-Hellman.
Default: 1024 bits
.. cpp:function:: bool hide_unknown_users() const
The SRP and PSK suites work using an identifier along with a
shared secret. If this function returns true, when an identifier
that the server does not recognize is provided by a client, a
random shared secret will be generated in such a way that a
client should not be able to tell the difference between the
identifier not being known and the secret being wrong. This can
help protect against some username probing attacks. If it
returns false, the server will instead send an
``unknown_psk_identity`` alert when an unknown identifier is
used.
Default: false
.. cpp:function:: u32bit session_ticket_lifetime() const
Return the lifetime of session tickets. Each session includes the
start time. Sessions resumptions using tickets older than
``session_ticket_lifetime`` seconds will fail, forcing a full
renegotiation.
Default: 86400 seconds (1 day)
.. _tls_ciphersuite:
TLS Ciphersuites
----------------------------------------
.. cpp:class:: TLS::Ciphersuite
.. cpp:function:: u16bit ciphersuite_code() const
Return the numerical code for this ciphersuite
.. cpp:function:: std::string to_string() const
Return name of ciphersuite
.. cpp:function:: std::string kex_algo() const
Return the key exchange algorithm of this ciphersuite
.. cpp:function:: std::string sig_algo() const
Return the signature algorithm of this ciphersuite
.. cpp:function:: std::string cipher_algo() const
Return the cipher algorithm of this ciphersuite
.. cpp:function:: std::string mac_algo() const
Return the authentication algorithm of this ciphersuite
.. _tls_alerts:
TLS Alerts
----------------------------------------
A ``TLS::Alert`` is passed to every invocation of a channel's
*proc_fn*. Most of the time, no alert was sent and so the alert will
be of type ``NULL_ALERT``.
.. cpp:class:: TLS::Alert
.. cpp:function:: is_valid() const
Return true if this alert is not a null alert
.. cpp:function:: is_fatal() const
Return if this alert is fatal or a warning alert
.. cpp:function:: Type type() const
Returns the type of the alert as an enum
.. cpp:function:: std::string type_string()
Returns the type of the alert as a string
TLS Protocol Version
----------------------------------------
TLS has several different versions with slightly different behaviors.
The ``TLS::Protocol_Version`` class represents a specific version:
.. cpp:class:: TLS::Protocol_Version
.. cpp:type:: enum Version_Code
``SSL_V3``, ``TLS_V10``, ``TLS_V11``, ``TLS_V12``
.. cpp:function:: Protocol_Version(Version_Code named_version)
Create a specific version
.. cpp:function:: byte major_version() const
Returns major number of the protocol version
.. cpp:function:: byte minor_version() const
Returns minor number of the protocol version
.. cpp:function:: std::string to_string() const
Returns string description of the version, typically "SSL v3" or
"TLS v1.n"
|